In the prior art, various types of materials have been proposed for use as liners in high pressure vessel or tank construction. Liner materials include metallic liners such as steel disclosed in U.S. Pat. No. 4,714,094. Alternatively, non-metallic liners such as polyethylene or polypropylene have been utilized as well as polyamides such as nylon 6, disclosed in U.S. Pat. No. 5,025,943.
However, metallic liners for pressure vessels are disadvantageous given their excessive weight. In applications where the pressure vessels function as fuel containers, the excessive weight results in lower fuel economies.
The above-mentioned thermoplastic-type liners also have drawbacks when used in high pressure vessels in connection with fuel storage. Typically, pressure vessels containing compressed natural gas (CNG) are subject to extremely high pressures, for example, 2,400 to 3,600 psi and large service temperature extremes, typically -40.degree. F. to +140.degree. F. Although non-metallic liner construction can offer the advantage of lighter weight and higher gas volumes than other types of tanks, some non-metallic liners suffer from a relatively high permeation rate of the gaseous contents of the pressure vessel. Since these types of thermoplastic resins are porous on a microscopic basis, they are subject to gaseous diffusion through the thermoplastic material, the diffusion resulting in gas loss from the high pressure vessel and a potential safety hazard. To reduce this permeation rate to an accepted level, the thickness of these liners must be increased resulting in reduced tank volume. Other of these prior art thermoplastic materials have low permeation but when subject to the high pressure and large temperature extremes required in fuel tank service, they fail in a brittle manner at the low end of the service temperature extreme (-40.degree. F).
Given the brittleness and unacceptable levels of permeability for the prior art thermoplastic liners, a need has developed to provide a thermoplastic liner having low permeability and sufficient flexibility or elongation to withstand the temperature extremes typically found in highly pressurized gas vessels.
In response to this need, the present invention overcomes the deficiencies in prior art materials by providing a thermoplastic liner made of a modified nylon 6 or nylon 11 material as a pressure vessel liner.
Nylon 11 marketed as Rilsan.RTM. B and manufactured by ELF ATOCHEM a division of Elf Aquitane of France, is known for use in natural gas pipelines given its superior mechanical and chemical properties over other plastic materials. Nylon 11 has been found to be resistant to stress cracking and degradation in soil environments. However, there is no recognition in the prior art concerning nylon 11 in the application as a high pressure vessel liner for gaseous fuel storage such as CNG with the associated mechanical and permeation requirements for this particular service application.
Modified nylon 6 marketed as Zytel.RTM. and manufactured by Dupont in the U.S.A. is also known and disclosed in U.S. Pat. No. 5,091,478, hereby incorporated in its entirety by reference. It is intended that the term "modified Nylon 6" encompass to all formulations encompassed by this patent for application according to the invention. This thermoplastic composition is disclosed for use in a wide range of molding, coating and adhesive applications including various automotive applications, wire and cable coating and high temperature adhesive applications. Again, the prior art does not teach the use of this thermoplastic composition in highly pressurized fuel tank gas liners as a liner material or its low permeability and sufficient mechanical properties for this particular service application.
In the prior art, it is also known to overwrap liners for pressure vessels with filaments in various configurations to improve vessel load resistance. In U.S. Pat. No. 3,969,812, a method of manufacturing an overwrapped pressure vessel is disclosed. The construction of this overwrapped pressure vessel is illustrated in FIG. 1 wherein the pressure vessel is designated by the reference numeral 1. Included in the overwrapped filaments are cylindrical or hoop windings 3 and helical or polar windings 5, 7 and 11. As disclosed in this patent, the cylindrical windings extend into the dome portion as shown by reference numeral 9. A final cylindrical layer 15 is provided which terminates at each of the dome transition points 17. The helical or polar windings are formed in overlapping relationship to form a laminate structure of the filaments around the outer surface of the liner 19.
U.S. Pat. No. 5,025,943 to Forsman also discloses a pressure vessel having a filamentary wound structure. In this patent, cylindrical or hoop windings are combined with helical or polar windings. In this pressure vessel, the cylindrical or hoop windings terminate at the cylinder-to-dome transition point.
U.S. Pat. No. 3,368,708 to Pflederer also discloses a filament wound tank design wherein the filaments are wound at helix angles in the range of 24.degree. to 191/2.degree. for optimal stress resistance. This patent does not disclose overwrapping the cylinder-to-dome transition.
U.S. Pat. No. 2,995,011 to Kimmel discloses a solid propellant rocket motor utilizing a fiberglass roving impregnated with an epoxy resin wound in a reverse spiral pattern over the propellant charge assembly. This single layer winding serves as a combustion or burning restricting material.
U.S. Pat. No. 4,714,094 to Tovagliaro also discloses an overwrapped gas-oil pressure accumulator. In this patent, low angle helical windings are used along the cylindrical portion of the accumulator due to the relatively large port diameter. The angle of the windings is approximately 55.degree. with respect to the longitudinal axis of the accumulator. The helical windings extend in the transition zone between the cap and the lining dependent upon the angle of the filaments therein.
However, these prior art designs are disadvantageous in failing to adequately strengthen the cylinder-to-dome transition area. In U.S. Pat. No. 3,969,812, the termination of the cylindrical windings is shown by reference numeral 17 in FIG. 1. To overcome these problems, prior art designs use excessive windings which add extra cost and weight to the pressure vessel. However, when the cylindrical windings 9 at essentially 90.degree. to the longitudinal axis of the pressure vessel cover the cylinder-to-dome transition area as shown in FIG. 1, the windings slip off the dome. This slippage makes it difficult if not impossible to overwrap and maintain these windings in a tight and adjacent fashion during pressure vessel manufacture. Invariably, the filaments slip down the cylinder-to-dome transition, thereby delaying the winding process or compromising pressure vessel integrity or rendering the vessel useless. To prevent this slippage, dams or other restraining devices are used on the exterior surface of the liner to prevent this slippage. However, these dams also add extra cost in manufacturing and produce a stress riser due to an abrupt change in section modulus which can diminish performance.
Moreover, limited slippage of the cylindrical windings 9 of FIG. 1 results in gap formation between adjacent windings. When helical windings overlap the spaced apart cylindrical windings, a void is formed in the overwrapped structure which provides a low strength area in the overall pressure vessel construction.
Thus, a need has developed to overcome the drawbacks of these types of overwrapping methods and liner construction. In order to overcome the deficiencies noted above in the prior art, the present invention provides a novel method of winding filaments in the helical and hoop directions. In particular, high angle helical windings are wound along the cylinder and through the cylinder-to-dome transition point at high angles, up to 88.degree. from the longitudinal axis of the pressure vessel. These high-angle helical windings eliminate the slippage of the cylindrical windings described above and also eliminate void occurrence and reduced strength properties in this transition area.